1. Field of the Invention
The present invention relates to a telecommunication cable equipped with at least one optical fibre. In particular, the present invention relates to a telecommunication cable equipped with at least one protecting layer optical fibre, especially suitable for indoor installations.
2. Description of the Related Art
An optical fibre generally comprises a core surrounded by a cladding, said core and cladding being preferably made of glass, and at least one coating. The combination of core and cladding is usually identified as “optical waveguide”. Usually, the coatings of the optical waveguide are two. The coating directly contacting the optical waveguide is called “first coating” or “primary coating”, and the coating overlying the first one is called “second coating” or “secondary coating”. Typically, said first and second coatings are made of a polymeric material, such as a UV-curable acrylate polymer.
Certain applications require the optical fibre to be further coated by a buffer coating provided over the at least one coating.
When said buffer coating is provided substantially in contact with the at least one exterior coating it is said to be a “tight buffer”. When said buffer coating is in the form of a tube having an internal diameter larger than the overall external coating diameter (the outer diameter of the optical fibre typically is of 240-250 μm), it is said to be a “loose buffer”. Depending on the difference between the fibre outer coating diameter and the buffer inner diameter, a loose buffer can be identified as “loose” or “near tight”.
Typically, a buffered optical fibre can be used as semi-finished component to form a cable in association with other components as required by the specific use to which the cable is intended. In some applications, when additional protection is not required, the buffered optical fibre can be used as such to operate as a cable.
Examples of these applications are indoor and premises installations, cable terminations, pigtails, patchcords and, more generally, those applications in which the optical fibre is subjected to repeated mechanical stresses because of recurring installation operations.
Optical fibres having a low-density region disposed around the fibre core, and preferably in the fibre outer cladding, may be tailored to provide robust bend-resistance. Generally, the low density region is an annular region comprising non-periodically, and in general randomly (i.e., irregularly located within the region), distributed holes or voids, to provide a single mode transmission. Herein, the term void may indicate empty holes, air-filled holes or bubbles containing gases trapped within them, and in general a defect having a refracting index significantly smaller than that of the surrounding matrix, and generally having a refractive index equal or close to 1. The pattern of voids leads to an effective lowering of the refractive index.
The above-mentioned type of optical fibres is called “microstructured” optical fibres. Microstructured optical fibres include deliberately-introduced defects, which generally are voids running longitudinally along the fibre axis. These voids may not extend the whole length of an optical fibre.
U.S. Pat. No. 7,397,991 relates to fibre optic cable having at least one optical fibre such as a microstructured bend performance optical fibre disposed within a protective covering. The protective covering such as a buffer layer and/or a jacket uses a bend radius control mechanism for protecting the optical fibre by inhibiting damage and/or breaking of the optical fibre. The jacket material used had a relatively high ultimate elongation (i.e., elongation before breaking) measured according to DIN 53504 (a German measurement standard), thereby providing a highly flexible fibre optic cable design. Jackets for fibre optic cables of the invention have an ultimate elongation that is about 500% or greater such as about 600% or greater, and even about 700% or greater. The PU (polyurethane) jacket material used had an ultimate elongation of about 800% along with a 300% tensile modulus of about 8.0 MPa
The patent application PCT/IB2007/002187 relates to a telecommunication cable equipped with at least one optical fibre coated by a tight buffer layer made from a polymeric material having an ultimate elongation equal to or lower than 100% and an ultimate tensile strength equal to or lower than 10 MPa. The combination of features of the polymeric material forming the buffer layer allows to obtain an optical fibre which is effectively protected during the installation operations and during use, and at the same time can be easily stripped by the installer without using any stripping tools, simply by applying a small pressure with his fingertips and a moderate tearing force along the fibre axis.
Applicant has observed that, in an optical fibre with a random-void distribution, the thickness of the void-containing region and the local density of the voids primarily determines the bend resistance of the optical fibre. Notably, bend resistance has been seen to be directly correlated with the product of the local void density and the area of the void-containing region.
The Applicant has faced the problem of providing optical cables particularly suitable for riser and horizontal indoor installations, e.g. in multi-floor buildings, (as known as “fibre-to-the-home” or FTTH application) with the aim of protecting the optical fibres contained therein during both the installation and the operation.
The Applicant noticed that the optical fibre can be subjected to mechanical stress during not only the installation and connection with the premises, for example in the extraction from a riser cable at a certain floor of a building, but also to static fatigue during the operation life, for example because of bending and/or winding in coils.
The Applicant has observed that the optical performance of the fibre is not the only element to be taken into account. When a cable or buffered fibre is bent around a corner for installation within a building or coiled on a small diameter, the fibre housed therein is maintained at the corresponding bending value for the cable life and this permanent deformation status can be source of unexpected mechanical breaks of the fibre (this phenomenon being generally addressed as “static fatigue”).
As static fatigue is intended failure of a component as a result of sustaining a heavy, continuous load.
Accordingly, even with fibres showing excellent optical performances in bent conditions, the minimum bending radius should be limited for mechanical reasons.
In addition, the Applicant has observed that bending the cable around relatively sharp corners or winding in small diameter coils would suggest the use of a buffer layer or sheath made of a “soft” material i.e. a material with very high compressibility, (or low elastic modulus); a material having high elongation at break seemed also desirable, both such conditions being usually found in the same material.
However, the Applicant has observed that in some typical uses of a cable housing a microstructured optical fibre a sheath, or a buffer layer, having a high elongation at break and low elastic modulus materials turns out to be a problem.
In particular, during the installation of a cable in a building the operator is in the need of peeling off the sheath or the fibre buffer layer, in order to apply a connector to the fibre, to make a mechanical splice, or a butt welding of the fibre or the like.
In such circumstances, a highly stretchable material would require some special tools to peel of the relevant layer, and an improper use of such tool may result in mechanical stress applied to the fibre.
To the contrary, a relatively stiffer material, with a low elongation at break allows the operator to peel off the buffer layer with the fingers or with a simple tool, with practically no stress applied to the fibre.